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The Dynamics of Mass Accretion of Vela X-1 Brady Ells, John Blondin Department of Physics, NC State University, Raleigh NC Introduction Vela X-1 is an x-ray binary star system consisting of a blue supergiant (HD 77581) and a neutron star. In an x-ray binary star system, two stars orbit each other closely enough for mass transfer to occur. When stellar wind radiated from the primary star becomes heated by the bow shock emitted by the compact star, it loses speed. This heating process allows more mass to be captured by the gravity of the compact star. When matter captured falls onto the star, the large amount of potential energy of the matter is released in the form of x-rays. Higher amounts of released potential energy translates to higher luminosity. An increase in energy transferred to the stellar wind causes an increase in mass accretion. Vela X-1 is characterized by its range of variability in its x-ray luminosity, making it difficult to model its mass accretion rate. The relationship between mass accretion and luminosity can be described by L = where R is the radius of the neutron star and dm/dt is the mass accretion rate. Method I started my research by taking an existing 1D model of a generic binary system and changed six parameters to more accurately portray Vela X-1. Next, I used my 1D model to investigate how the variables cak_k and abbott affect the terminal wind velocity and mass loss rate of Vela X-1. cak_k and abbott are used in an attempt to average the radiative driving force over multiple absorption lines. I graphed the results (shown right), which allowed me to visualize the relationships I was studying. I intend to incorporate the data found into a 3D model of a binary star system assuming an adiabatic wind. My next step will be to address the impact that photoionization of the stellar wind, and its x-ray shadow, have on the mass accretion. Photoionization occurs when electrons escape their orbits when energy is transferred from the x-rays to the stellar wind. I will also include x-ray heating feedback from the neutron star in my code. X-ray heating is the temperature increase of the stellar wind when it comes in contact with the bow shock of the neutron star. Property Value Temperature of supergiant 2.7e4 K Orbital period 8.964 days Mass of supergiant 23.5M Mass of neutron star 1.88M Separation distance 53.4R Supergiant luminosity 3.8e5L Wind Data These were created using a 1D model of a radiation-driven stellar wind from a primary star, adapted to Vela X-1 parameters, prior to its interaction with the gravitational field of the neutron star. Objective My objective is to create a 3D model of Vela X-1 that accurately measure mass accretion rate. This will be accomplished by finding a self-consistent model of luminosity and mass accretion. I will create this self-consistent model by determining the cause of the varying luminosity by reviewing past literature. I will be using VH-1 to run simulations using a 3D model adapted to fit Vela X-1 parameters (shown below). Primary star Neutron star Mass transfer The bow shock of the stellar wind from the primary star of Vela X-1 GM dm x R dt A 3D image produced on EnSight 10 of a binary system. The primary star is losing mass to the neutron star through the stellar wind.
